The invention relates to depositing dots of adhesive at selected locations on the surface of a printed circuit board by sensing a reference spacing, between an adhesive dispensing nozzle tip advancing toward the substrate and each selected location on the substrate, and halting advance of the tip at a preferred distance from each selected location. The invention also may relate to spacing of the tip of a vacuum spindle of a pick and place machine for handling surface mountable device (SMD) components.
Prior art devices generally include those which touch the depositing tip to the selected location on the substrate surface at which material is to be deposited and retract the tip away from the surface by a specific amount in order to attain a preferred spacing therebetween. With devices which sense contact with the surface by a reactive force such as disclosed in U.S. Pat. No. 4,661,368, calibration difficulties are encountered when used with flexible substrates, since contact with the substrate can cause it to flex before a required amount of reactive force is sensed via the tip. Thus, subsequent retraction of the tip, based on the sensed position of the surface, can result in insufficient spacing therebetween. Additionally, touching the board with the tip can cause jarring or movement of components and nicking of the fragile surfaces of some substrates.
Other prior art surface locating devices utilize so called "outriggers" in sensing the appropriate spacing between a tip and a substrate by engaging and locating the substrate at a location other than the exact place at which deposition is to take place. Thus, a much larger "footprint" on the circuit board, involving the depositing tip and the sensing outrigger, is required. Additionally, this prior art does not compensate for circuit board warpage or surface waviness at the specific location at which depositing is to take place, since it senses a location on the substrate which is spaced from the specific location at which deposition is to occur.
Thus, it is an objective of the invention to avoid contact with the board by the sensor while providing exact spacing between a dispensing tip or the like and each selected location on the board at which material is to be deposited.
Further, it is an objective of the invention to avoid contact of the board by the dispensing tip and thus reduce the time necessary in retracting the tip from the substrate prior to depositing.
Additionally, it is an objective of the invention to avoid damaging the surface of the substrate and jarring or moving other components already mounted on the substrate.
Also, it is an objective of the invention to sense the spacing between the substrate and depositing tip at the exact selected location for each deposited dose or component so as to minimize the footprint on the substrate.
Still further, it is an objective of the invention to avoid the calibration difficulties inherent with "contact" type sensors when used with flexible substrates.
Additionally, it is an objective of the invention to avoid spacing inaccuracies caused by board warpage or waviness inherent in use of devices which sense locations on the substrate other than the exact location where depositing is to take place.